Free Certificates from Comodo

Comodo has a form for requesting a certificate. They are free, good for a year, can be picked up using most browsers, and can be 512, 1024 or 2048 bits long. (Less bit length choice is offered to some browsers. JavaScript must be enabled to request a certificate, and JavaScript issues can interfere with bit length choice.) They changed their procedures in October 2010 and new certificates no longer contain a personal name, just your email address. Only one Secure Email Certificate can be issued per Email Address.

To obtain certificate from an authority

Browsers such as Firefox, MSIE, Opera, Safari and so forth contain cryptotools capable of generating public/private keypairs. When signing up for a certificate with an authority, their website triggers your browser to create a keypair and transmit to them the public key, which is then certified. For this reason, when you return to pick up your completed certificate (typically a few minutes later), it is mandatory that you do so with the same browser on the same computer. You will otherwise not possess the private key necessary for pickup.

It will then still be necessary to export the resulting new key and certificate to a regular but password protected file that can then be imported into Thunderbird's certificate store. The CA's and/or your browsers help files should explain how to export your new certificate and keys.

To export it as a .p12 (Personal Information Exchange File) file using Firefox go to Tools -> Options -> Advanced -> Encryption -> View Certificate and select the certificate. Then press the backup key. You will be prompted for a password which you will have to enter when you import the file.

To import the file into Thunderbird, use "Tools -> Options -> Advanced -> Encryption -> View Certificates -> Import". You will be prompted for the password you used when you exported (or backed up) the file.

Once you have imported your certificate into Thunderbird, it will be available for pairing with one or more accounts in Thunderbird using Tools -> Account Settings -> Security -> Select.

Be thoughtful about whether to select to "digitally sign all messages by default". Institutional firewalls may protect their own security protocols and break your cryptographic signature, leaving your recipient with all kinds of warnings about the message being invalidly signed. As S/MIME usage is still not widespread, most people still don't know how to interpret this. A broken signature will probably seem worse to them than receiving a message with no crypto signature at all, even though the contents are identical in both cases.

Webmail users will see an unreadable attachment which can raise similar questions.

Self-signed certificates

You may use a personally self-signed certificate in Thunderbird. However, since these certificates are not signed by an approved certificate authority, the certificate will not be trusted by other computers or people unless they add the self-signed certificate to their list of certificate authorities. Personally self-signed certificates are generally only useful for testing or for exchanging information with people you already know and trust.

Self-signed certificates in Mac OS X 10.4

You can create your own self-signed certificate using the Keychain Access application's Certificate Assistant. To export your certificate as a PCKS12 file for import into Thunderbird, click "My Certificates" in the Keychain Access window. Select your self-signed certificate. Then from the menu bar select "File -> Export". You will be asked for a password to protect this file. This is the password you will require when importing the certificate into the "Your Certificates" tab of Thunderbird after entering your master password.

To export your certificate as a ".cer" file for use as a certificate authority, select "Certificates" in the Keychain Access window. Select your self-signed certificate. Then from the menu bar select "File -> Export". Be sure ".cer" is selected as the appropriate file type in the save dialog.